The recently developed Interferometric Synthetic Aperture Radar (InSAR) is technology that has been successfully used to two-dimensionally observe ground surface displacement within a broad area of a few tens of km2 or broader with a measurement sensitivity ranging from a few centimeters to a few millimeters.
For past 15 years, InSAR has been used to observe earthquakes, volcanos, glaciers, landslides, groundwater pumping, the subsidence of reclaimed land, the subsidence of abandoned mines, etc. The data generated by InSAR is called a radar interferogram. Precise ground surface displacement can be observed from such a radar interferogram in the line-of-sight (LOS) of an antenna.
However, this technology can observe only one-dimensional ground surface displacement in the direction of observation, and thus has a limitation in terms of the three-dimensional analysis of the mechanism of ground surface displacement in the case of seismic activity, volcanic eruption, a landslide, the movement of a glacier, or the like. Accordingly, many researchers have actively conducted research for two- or three-dimensional ground surface displacement in order to overcome the above disadvantage.
As an example, a method of determining ground surface displacements in a ground range direction and in a direction vertical with respect to a ground surface from a plurality of images having different paths using InSAR is known. However, this has the disadvantage of being unable to observe ground surface displacement in a direction of flight.
Furthermore, a method using a correlation coefficient between two images or various images was proposed, and has been widely used to observe three-dimensional ground surface displacement. However, since the accuracy of measured ground surface displacement is very low, this method can be applied only to a ground surface displacement of tens of cm or longer.
In contrast, MAI (multiple aperture SAR interferometry), recently developed and designed to improve the observational accuracy of ground surface displacement in a direction of flight, generates a forward-looking interferogram and a backward-looking interferogram through split-beam InSAR processing and generates an MAI interferogram from these two different interferograms. This method exhibits significantly higher accuracy than the method using a correlation coefficient. It is known that when an ERS SAR satellite image is used, it is possible to observe ground surface displacement in a direction of flight with an accuracy of about 8 cm at a coherence of 0.6. Furthermore, it is known that when an ALOS PALSAR satellite image is used, it is possible to observe 3D ground surface displacement in the east and in a vertical direction with an accuracy of about 2 cm and in the north with an accuracy of about 4 cm or lower.
Accordingly, Korean Patent 10-1111689 entitled “Method and Apparatus for Extracting 3D Ground Surface Displacement” proposes a technology that extracts a ground surface displacement in an LOS direction and a ground surface displacement in a direction of flight using an SAR interferogram and an MAI interferogram and extracts a 3D ground surface displacement from these ground surface displacements.
However, this related technology has the advantage of being able to observe accurate ground surface displacement using an SAR interferogram, but has a disadvantage in that an SAR interferogram may occasionally involve serious distortion, such as a striped pattern effect, due to an ionospheric change. That is, although this related technology proposes an attempt to increase accuracy by eliminating flat-earth phase, altitude phase, and residual phase attributable to matching error in the step of generating MAI data, this related technology cannot fundamentally overcome a distortion effect attributable to an ionospheric change.
Although such distortion appearing in an SAR interferogram should be corrected, a method capable of efficiently eliminating ionospheric distortion has not been developed yet because of the occurrence of a precision-related problem.
Therefore, there is a demand for a method that is capable of fulfilling accuracy while correcting an ionospheric distortion phenomenon that appears in an SAR interferogram.